Add a benchmark for String#split with -1 limit

The CSV library calls split(",", -1) to split each line into columns,
so this benchmark should help measure the performance problems around
CSV operations in Rubinius.
Avoid constructing a Regexp when splitting a String with negative limit

When splitting a string using a String pattern, we don't want to turn it
into a Regexp unless we absolutely have to. Prior to this change,
passing a negative limit into String#split would have forced this
conversion and incurred a high performance cost as a result.

Running the benchmark for this particular case shows a three-fold
improvement.